1. Field of the Invention
The invention relates to the field of batteries and, in particular, to thermal batteries and methods of recharging such batteries.
2. Description of Related Art
Thermal batteries are noted for their extremely high discharge rate capability (high power delivered for short periods of time) and they generally have a very long storage life. A prior art thermal battery typically comprises of a plurality of cells, each comprised of a positive electrode and a negative electrode spaced apart by an inactive separator material containing an electrolyte that becomes ionically conductive when heated above the electrolyte melting point. A combustible material is disposed between the cells and in contact therewith for supplying heat to the electrolyte. The combustible material is typically a mixture of a finely divided oxidizer, such as potassium perchlorate, and a finely divided metal, such as iron powder. When activated by an external means, the combustible material will exothermically react to liberate a considerable amount of heat and sinter into an electronically conducting element between the cells. Thus this material contributes greatly to the weight of the battery. Upon ignition, the combustible material heats the electrolyte to a temperature wherein it melts. An explosive squib is used to initiate the combustion process.
At this point, the thermal battery will produce electrical power, generally for only a short period (at most seconds to minutes depending on the external load and thermal insulation design). Thus they have general application as a back-up power supply. In addition, they have application in non-emergency systems. On launch vehicles, weapons systems, and spacecraft there is often a need for large amounts of electrical energy for short periods. The weight of conventional batteries would be prohibitive. However, thermal batteries weighing as little as a few pounds provide more than adequate performance. An extreme example is that two fifty pound thermal batteries providing a nominal 270 volts and 450 amperes for three minutes can replace 2500 pounds of conventional lead acid batteries, in an application that might be used on the space shuttle. Typical thermal batteries are disclosed in U.S. Pat. No. 4,041,217 xe2x80x9cThermal Battery With Metal Oxide Heating Compositionxe2x80x9d By W. H. Collins and U.S. Pat. No. 3,898,101 xe2x80x9cThermal Batteryxe2x80x9d by D. M. Bush, et al.
As previously mentioned, such thermal batteries are usually short lived. Once the heat generating chemicals are exhausted, the battery begins to cool down. Over a rather short period of time, because of the cooling down, the electrolyte can become inactive even though a considerable amount of stored chemical energy may remain within the battery. However, many such batteries are drained of their charge, before the electrolyte becomes inactive. It is these later conditions that the subject method is directed at.
Thus, it is a primary object of the invention to provide a method of recharging a pyrotechnically activated thermal battery.
It is another object of the invention to provide a method of recharging a pyrotechnically activated thermal battery while the electrolyte within the battery is still active.
The invention is a method of recharging a thermal battery after it has been activated and at least partially discharged and the battery is still at a temperature where the electrolyte is still active. The method comprises the step of at least partially recharging the battery prior to the battery temperature reaching a level where the electrolyte is non-active. Additionally, a layer of thermal insulation may be placed about the battery. Furthermore, an electrical heating coil may also be placed about the battery to maintain the battery at operating temperatures.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.